1. Field of the Invention
The present invention relates to a non-destructive inspection device in which, when a defect of a member to be inspected (hereinafter, such a member is often referred to as inspection member) having a complex shape configured by combining planar and curved faces with one another is inspected in a non-destructive manner, the distance (lift-off) between an inspection probe and the inspection member is controlled to be always constant, thereby improving the inspection accuracy.
2. Description of the Related Art
As the advancement of downsizing and performance of a mechanical structure or a device, a stress which is applied as a load to a structure is increasing. Therefore, even when a defect is small from the viewpoint of a field of a material strength, stress concentration produces a large influence. As a result, it is requested to surely detect even a small defect (impact mark, crack, damage, grinding burn, or the like).
Conventionally, a device shown FIG. 5 is known as a defect inspection device which satisfies such a requirement. In the device, an AC current is supplied to a coil 110 to induce eddy currents in an inspection member 11 which is opposed to the coil. A defect is detected in a non-destructive manner on the basis of a phenomenon in which the amplitude and phase of the eddy currents are changed depending on the dimensions, kinds of the defect, the permeability, the conductivity, and the like of the inspection member 11, i.e., a change of an impedance of the coil 110. In FIG. 5, xcfx86xe2x80x2 indicates magnetic fluxes generated by the eddy currents. FIG. 6 is a diagram of eddy currents in the case where the inspection member 11 has no defect. By contrast, in the case where the inspection member 11 has a defect, i.e., the case where a passage of a conductor is bent to change its shape as shown in FIG. 7, also eddy currents are changed in accordance with the change, so that the defect of the inspection member 11 is inspected in a non-destructive manner.
It is known that, when the coil 110 is wound into the absolute type as shown in FIG. 8, such a defect inspection device based on eddy currents is suitably used for detecting changes in all of the dimensions, kinds of the defect, the magnetic permeability, the conductivity, and the like. It is known also that, when two coils are wound in the differential type, a crack can be detected more sensitively.
An inspection member which is to be inspected by such a non-destructive inspection device often has a complex shape configured by combining planar and curved faces with one another. When an inspection member having such a complex shape is to be inspected, a moving mechanism having degrees of freedom in three or more axes is used, and inspection is performed while tracing the shape of the inspection member by interpolating the three axes. In a method of the conventional art, coordinates of an inspection probe for scanning the inspection member are preset, and inspection is performed in accordance with the preset coordinates.
Therefore, the distance (lift-off) between the inspection probe and the inspection member is affected by, for example, setting errors of the inspection probe and the inspection member, and the tolerance of the inspection member, and hence the lift-off is varied. When the lift-off is changed by scanning of the inspection probe, setting errors of the inspection probe and the inspection member, the tolerance of the inspection member, and the like, eddy currents in the inspection member due to magnetic fluxes generated by the coil in the inspection probe is changed, so that the impedance of the coil in the inspection probe is largely changed. The impedance change of the coil in the inspection probe is larger than that due to a defect to be detected. When the lift-off is varied, therefore, there arises a problem in that the detection accuracy of the inspection is not stable.
When inspection using an inspection probe is performed while setting based on design values is unchanged, a dimension difference of several tens of p.m is caused in the lift-off by the tolerance error of the workpiece, the error in chucking of the inspection member, the error in setting of the inspection member, and the like. An output signal of a non-destructive inspection device has relationships such as shown in FIG. 2 with the lift-off. When the lift-off is changed, therefore, also the level of the output signal is largely changed.
FIGS. 9 and 10 show relationships between the lift-off and the detection signal, and the state of a defect in the detection signal. When inspection is performed with placing an inspection probe 20 in position A (too close), B (normal), or C (too remote) with respect to an inspection member 21, the level of the detection signal (output voltage) V is changed as indicated by VA, VB or VC. Namely, as the lift-off is larger, the level of the output voltage V is lower, and, as the lift-off is smaller, the level of the output voltage V is higher. An inspection area ARA of the output voltage VA shows a signal corresponding to a defect, an area inspection ARB of the output voltage VB shows a signal corresponding to the defect, an inspection area ARC of the output voltage VC shows a signal corresponding to the defect, and xcex94VA, xcex94VB, and xcex94VC show the levels of the signals corresponding to same defect, respectively.
Conventionally, in an inspection member, the requested accuracy of the size of a defect is not very high. Even when the absolute value of the output signal is somewhat varied in degree, therefore, a defect can be detected because the level difference between outputs from a defect detection section are large. By contrast, with respect to a small defect, the output signal is low in level. When the lift-off is large and the level of the output signal is low, therefore, the output signal is buried in noises and cannot be detected. When VB greater than VC and xcex94VB greater than xcex94VC in FIG. 10, for example, it is impossible to judge which defect is larger.
On the other hand, when the distance between the inspection probe and the inspection member is too small, there is a fear that the probe may be in contact with the member. Usually, as the surface area where an inspection probe is opposed to an inspection member is larger, the detection accuracy is higher, and an output of a sufficient level cannot be obtained unless the probe and the member are somewhat closer. In the case where an inspection member having a complex shape configured by combining a planar face (straight line) and a curved face (curved line) with one another, a concave curved or spherical face, a cylindrical face, or the like is to be inspected, when an inspection probe is planar or larger in radius of curvature than the inspection member, therefore, a state may be caused in which the inspection probe is made excessively closer to the inspection member to be in contact therewith.
In such a non-destructive inspection device, usually, defect judging means makes a judgement in either of the following manners: (1) if the difference between a detection signal obtained from an inspection member with respect to a reference level is larger than a predetermined value, the inspection member is judged failure; and (2) if the difference between a detection signal with respect to a reference level is smaller than a constant value (for example, 20%), the inspection member is judged acceptable, and, if not, the inspection member is judged failure. However, such means has a problem as follows. In (1) above, when the probe position is excessively remote as in the case of position C shown in FIG. 9 and the signal level is low, the inspection member is judged acceptable in spite of failure. By contrast, in (2), the inspection member is judged failure not only when the probe position is excessively remote as in the case of position C with the output level is low, but also when the probe position is excessively close as in the case of position A with the output is high.
The invention has been conducted under the above-mentioned circumstances. It is an object of the invention to provide a non-destructive inspection device in which inspection can be stabilized by correcting a change of the lift-off due to setting errors of an inspection probe and an inspection member, or the tolerance of the inspection member, the time period required for the correction can be minimized so as to suppress an increase of the inspection time period, and an influence of the change of the lift-off can be minimized without adding a distance sensor or the like.
The invention relates to a non-destructive inspection device which detects in a non-destructive manner a defect of a member to be inspected, based on a change in magnetic fluxes due to eddy currents that are generated by an inspection probe having a coil. The object of the invention can be attained by comprising: a driving section which adjusts a position of the inspection probe; and measuring device for, based on a detection signal of the inspection probe, measuring a lift-off between the inspection probe and the member to be inspected, and controlling the driving section in accordance with a result of measurement of the measuring device, whereby a control of making the lift-off constant is enabled.
The object of the invention can be attained more effectively by winding the coil into the absolute type, and outputting a detection signal of the inspection probe as an absolute value, or by configuring the coil as two coils which are wound in the differential type.